1. Field of the Invention
The invention relates to a device for noncontact intermittent contact scanning of a surface with an adjustment unit and a sensor with a tuning fork which has two spring tongues connected by a base part and which is arranged such that the two spring tongues are oriented essentially parallel to the surface to be scanned, and with a tip attached underneath to the front end of the lower spring tongue, and with the upper tongue attached to a mount the tuning fork mount assembly having a quality of at least roughly 1000 in air for a vibration perpendicular to its longitudinal axis and perpendicular to the surface to be scanned. The invention also relates to a process for noncontact scanning of a surface by means of a device with a sensor with an extended spring element and a tip attached thereto, the sensor being set into a resonant vibration, at least one vibration parameter being acquired as the vibration signal and the change of the signal which results from the action of the force between the tip and the surface to be scanned being used to control the distance between the surface to be scanned and the tip. Possible applications include scanning force microscopy and profilometry.
2. Description of Related Art
Scanning force microscopy is based on scanning a fine tip over a surface (in the x and y direction), by controlling the distance to keep constant the force acting between the tip and surface and to acquire an image from the vertical movement (movement in the z-direction) of the tip. Imaging is determined by the interaction of this tip with the surface. Basically, it is distinguished between an imaging mode with repulsive and attractive interaction between the tip and the specimen. When a tip approaches a surface, the force between the tip and specimen is first attractive. As soon as the tip and specimen "touch," the force is repulsive. This force is measured by the fine tip being mounted on a spring element or a leaf spring and the bending of this leaf spring being measured.
When the tip is scanned in the repulsive mode (this mode is used primarily in profilometers) the tip is worn away with time, in the attractive mode it remains sharp for a long time. According to T. R. Albrecht (T. R. Albrecht et al, J. Appl. Phys. 69, 668 (1991)) the attractive mode has advantages over the repulsive mode, because chemical bonding between the tip and the specimen is prevented and the tips are not worn off. In doing so, the leaf spring is excited to natural vibration by a piezoelement. The frequency is given by: EQU f.sub.0 32 1/(2.PI.)(k.sub.0 /m).sup.0.5 (Eq. 1)
where k.sub.0 is a spring constant and m is the effective mass. The interaction between the tip and surface yields a new effective spring constant EQU k.sub.eff =k.sub.0 +k'. (Eq. 2)
The tip-surface interaction results in a negative k'; thus, the new vibrational frequency becomes less than the eigenfrequency of the leaf spring. The frequency shift thus offers a measure for the average distance between the tip and surface and can be used to acquire an image (so-called FM mode). The vibrating tip is scanned over the specimen and the height z is adjusted such that the frequency shift remains constant.
If first we examine the attractive interaction between a spherical tip with radius R and a plane surface at distance z, according to J. Israelachvili ("Intermolecular and Surface Forces", Academic, London 1985) a force F(z) is given by: EQU F(z)=AR/(6z.sup.2) (Eq. 3)
where A is the co-called Hamaker constant, a material constant which is dependent on the material of the tip or surface. For solids it is roughly 10.sup.-19 J. The interaction constant k' is then the derivation of the force according to the distance, explicitly: EQU k'=-AR/(3z.sup.3) (Eq. 4)
The relationship between the frequency shift and distance is then given for k' &lt;&lt; k by: EQU .DELTA.f/f.sub.0 =0.5k'/k.sub.0 =-AR/(6k.sub.0 z.sup.3) (Eq. 5)
The frequency shift increases steeply as the distance decreases. When the tip is too far from the surface, the error signal is very small; it lasts until the control deviation is corrected On the other hand, if the tip is too near the surface, the error signal is very large, and the control circuit can oscillate.
For thermodynamic reasons, the measurable force gradient is not optionally small. Albrecht et al. (T. R. Albrecht, P. Gruetter, D. Home, and D. Rugar), J. Appl. Phys. 69, 668, 1991) have computed the measurable force gradient: EQU k'.sub.min =((4k.sub.0 k.sub.B TB)/(.pi.f.sub.0 A.sub.0 Q)).sup.0.5) (Eq. 6).
(k.sub.0 is the spring constant of the detector in N/m, k.sub.B is the Boltzmann constant in J/K, T is the temperature in Kelvin, B is the bandwidth of the frequency analyzer in Hz, f.sub.0 is the eigenfrequency in Hz, A.sub.0 is the vibration amplitude in m, Q is the quality). These thermodynamic factors yield a complication for measurement of the force gradient: the vibration amplitude cannot be made optionally small. The average tip-surface distance cannot become smaller than the vibration amplitude. Therefore, in practice, for the optimum vibration amplitude, a middle way must be found between the noise of frequency measurement at too small an amplitude and too large an average distance at too large an amplitude.
For minimally attainable resolution, besides Equation 6 there are two other criteria which relate to the relationship between the spring constant of the force sensor and attainable resolution. According to Equation 4, the tip-surface interaction results in a negative k' so that the vibration frequency due to the interaction becomes smaller than the eigenfrequency of the spring element. It is important for operation that k' must be smaller than k.sub.0, otherwise the tip snaps onto the surface and can no longer vibrate freely. The lateral resolution of the microscope is of the magnitude of the working distance d, i.e. at a given resolution .lambda., a barrier arises for the force constant (stability condition): EQU k.sub.0 &gt;12AR.lambda..sup.-3 (Eq. 7)
In particular, at a tip radius of 100 nm and a resolution of 1 nm, i.e. roughly two atomic diameters, it is found that the force constant of the spring element must be greater than 120 N/min. This value is computed from Equation 4. The second criterion relates to an upper barrier for the force constant. The interaction between the tip and surface deforms the latter. Assuming that the force application to the surface via a hemisphere with radius .lambda./2, in the volume under the surface, causes a strain s, the spring constant of the surface can be defined as: EQU k.sub.surface =2.lambda.E (Eq. 8)
where .lambda. is the resolution and E is the elasticity constant (for steel, for example, it holds that the value of E equals 2.times.10.sup.11 N/m.sup.2). When the spring constant of the surface is greater than k.sub.0, the surface bulges more strongly than the spring bends. For a desired resolution of 10 nanometers, k.sub.0 should not be greater than 4000 N/m.
A generic device for noncontact scanning of a surface and a generic process for noncontact scanning of a surface of the type to which this invention is directed are known, for example, from F. J. Giessibl, Science 267, 68 (1995), Y. Sugawara et al, Science 270, 1646 (1995) and M. Tortonese et al, Appl. Phys. Lett. 62, 834 (1993).
The attractive mode was operated so sensitively that, with it, for the first time atomic resolution could be detected on a semiconductor (F. J. Giessibl, Science 267, 68 (1995), Y. Sugawara et al, Science 270, 1646 (1995). This mode is much more complex to operate than the repulsive mode. The interaction constant k' is strongly dependent on the working distance. The frequency shift was used directly as the control variable. Thus, the loop gain of the frequency shift-distance control circuit depends, likewise, sensitively on the working distance. In the FM mode, this leads quickly to instabilities, and thus, to unreliable operation.
The bending of the lead spring was to date measured by a light beam which is directed at the spring and reflected or by an applied resistance which changes its value by bending (piezoresistive effect) orby a voltage produced by bending (piezoelectric effect). To do this, force springs microfabricated from silicon are used with spring constants around 10 newton/m and tip radii of curvature of a few nanometers (M. Tortonese et al. Appl. Phys. Lett. 62, 834 (1993). The springs are excited to vibrate by piezoplates.
Here, the disadvantage is that these known arrangements require an almost atomically sharp tip in order to achieve atomic resolution. Slight contact of the tip with the surface to be scanned can increase the tip radius such that stable operation is no longer possible. In addition, for good operation the mechanical contact between the force sensor and piezoplates must be very solid. But, this is often a problem, so that reliability suffers and noise is increased. In addition, the mechanical connection causes a phase shift which is hard to predict. Therefore, in the control electronics, the phase shift must be adjustable; this means additional cost. Another disadvantage is that these sensors can only be used in a vacuum because the mechanical quality of these sensors in air is too small to achieve high resolution (Eq. 6).
Dransfeld et al. (U.S. Pat. No. 5,212,987) describe a force sensor based on tuning forks where the sensor is mounted at the base of the fork. The mass of the tip on one tongue is compensated by putting a counterweight on the other tongue. The disadvantage of this technique is that it is difficult and expensive to tune the counterweight. Also, the arrangement is detuned when the tip interacts with the sample, causing a severe loss in quality factor upon interaction with the sample (see Eq. 6)
K. Bartzke et al., International Journal of Optoelectronics 8, Nos. 5/6, 669-676, 1993, discloses a sensor in which the spring element of the sensor consists of a quartz rod 2 mm long which is produced lithographically from a quartz plate and which has a spring constant in the longitudinal direction of the rod of roughly 100,000 N/m. On the face surface of the rod, by means of electronic beam induced deposition in a scanning electron microscope, a tip 3 microns long with a diameter of 300 nm is applied. The sensor is attached to the piezoelectric adjustment unit, the quartz rod being oriented perpendicularly to the surface to be scanned. The quartz rod forms an oscillator which is piezoelectrically excited by an external voltage signal and which, in the longitudinal direction of the rod, vibrates with an amplitude of a few nanometers at a resonant frequency of 1 MHz. The sensor is operated in a mixed mode which is between a purely attractive and purely repulsive mode. The change of vibration phase which results from the interaction of the tip and surface relative to the excitation signal is used for controlling the distance of the tip.
The disadvantage in this arrangement is that outside excitation requires additional components and implies the use of a vertical quartz rod, that the stiffness of the spring in the x-y plane is much less than in the z-direction, i.e. in a lateral force component, the sensor is deflected parallel to the plane. Another disadvantage is the extremely high spring constant (see Eq. 8).